1. Field of the Invention
The present invention relates to off-chip drivers, and more particularly, to an off-chip driver that can compensate for variations in temperature, process, and supply voltage.
2. Description of the Prior Art
An off-chip driver is a well-known circuit in the field of electrical technology. A typical off-chip driver (OCD) consists of at least a pre-driver and a final driver, both controlled by a supply voltage. The purpose of the pre-driver is to control when and how the final driver stage is turned on or off. The turned-on and turned-off rates of the final driver then determine the slew rate of the output signal. Slew rate (change in voltage over time (V/ns)) of the output signal is an important indicator of the quality of the OCD. It is desirable that slew rate stays within a certain range across all conditions. Variations in process, temperature, or voltage supply (PVT) can cause slew rates to fluctuate. There are therefore various methods utilized to control slew rate.
There are three main methods used to control the slew rate of an OCD: time domain, constant current mirror, and constant voltage methods. The time domain method uses delay stages. The final driver is divided into a plurality of branches, each controlled by a separate pre-driver. Delay stages are built into each pre-driver in order to control the turned-on and turned-off rate of each branch of the final drivers, which helps to reduce the variation of slew rate. A second method uses current mirrors to put constant currents in the pre-drivers, so that the slew rate of the pre-drivers and subsequently the slew rate of the output signal have less variation. Another conventional method utilizes voltage regulators to ensure that a constant voltage can be provided to the gates of the pre-driver of the OCD, regardless of changes to the supply voltage.
The problem with the constant current and constant voltage methods is that these methods do not compensate for changes due to variations in supply voltage, temperature or process. In the time delay method, due to large variations in the delay at different process, voltage and temperature, the slew rate alters considerably with these variations. Therefore, a slew rate control method that can compensate for all PVT variations is required.